Solenoid with multi-rate return spring

ABSTRACT

A multi-rate return spring optimizes solenoid operational efficiency by providing a spring restoring force versus deflection characteristic more nearly matching the inverse-square solenoid stroke versus force characteristic. The spring may be fashioned from two or more contiguous sections having progressively greater winding pitches such that the resulting multiple restoring force rates compositely more nearly match the solenoid force characteristic over a predetermined stroke.

United States Patent Campbell [451 May 23, 1972 [54] SOLENOID WITHMULTl-RATE RETURN SPRING Melvin 1.. Campbell, Marion, lowa Collins RadioCompany, Cedar Rapids, Iowa Apr. 27, 1911 Inventor:

Assignee:

Filed:

Appl. No.:

References Cited UNITED sTArEs PATENTS 3,207,961 9/1965 Lohr ..335l274X'SPRING RESTORATION FORCE '6 3,307,]30 2/1967 Camp ..335/258X PrimaryExaminer-George Harris Attorney-Richard W. Anderson and Robert J.Crawford ABSTRACT A multi-rate return springioptimizes solenoidoperational efficiency by providing a spring restoring force versusdeflection I characteristic more nearly matching the inverse-squaresolenoid stroke versus force characteristic. The spring may be fashionedfrom two or more contiguous sections having progressively greaterwinding pitches such that the resulting multiple restoring force ratescompositely more nearly match the solenoid force characteristic over apredetermined stroke.

7Clalms,3l)rawlngllgures Patented May 23, 1972 3,665,353

, SOLENOID FORCE LOAD SPRING RESTORATION FORCE FIG. I

I 9 CO'LS I6 5 cows l6a I 1 b I 55 I FIG. 2

.030 0 60 .093 .I25 I60 STROKE (INCHES) .250 .280 .3l9 .343 .375 .4IOSPRING LENGTH (INCHES) FORCE (GRA PIC-3.3

INVENTOR MELVIN L. CAMPBELL BY M AGENT SOLENOID WITH MULTl-RATE RETURNSPRING This invention relates generally to electrical solenoids and moreparticularly to an improved solenoid construction including a multi-ratereturn spring by means of which a large increase in usable work may beobtained from a solenoid without any increase in wattage.

Solenoids normally include return springs such that the solenoidplunger, upon electrical release, is forcefully and rapidly returned tohome position. Known spring return solenoids include a single ratereturn spring by means of which a linear rate as concerns springrestoring force versus spring deflection is realized. Thus, a particularspring restoring force versus spring deflection rate must be chosen suchthat throughout the stroke of the solenoid plunger the spring restoringforce characteristic as a function of spring length does not cross over"the solenoid forcestroke characteristic. Obviously, if during anyportion of the stroke the spring restoring force equals or exceeds theforce developed by the solenoid winding on the plunger, the solenoidplunger cannot be positioned over its intended stroke. Single ratereturn springs additionally impose restrictions and/or designcompromises to approach spring preload and full stroke spring returnenergy design specifications for a given application.

If solenoids had linearcharacteristics as concerns stroke versus forcedeveloped, a single rate return spring design .could efiicientlymaximize the full stroke spring return force for any given solenoidapplication. Solenoids, however, exhibit an inverse-square relationshipas concerns stroke versus force and this relationship is characterizedby a pronounced knee. Thus, for any given single rate return springdesign, a compromise must be made between maximizing the spring returnforce at the full stroke position of the solenoid core and avoiding acrossover in the spring and solenoid force-displacement/deflectioncharacteristics.

In the interest of optimizing the efficiency of any given spring returnsolenoid and maximizing the usable work obtained, it is desirable thatthe spring restoring force versus deflection characteristic match thesolenoid stroke versus force characteristic as closely as possible. Asingle rate restoring spring cannot be designed to match the inherentinversesquare characteristic of a solenoid.

Accordingly the object of the present invention is the provision of animproved spring return means for a given wattage solenoid by means ofwhich the spring return force characteristic is caused to morenearly'match the solenoid force characteristic and by means of which theoperating efficiency is improved and a large increase in usable work isobtained without any increase in wattage required for a givenapplication.

The present invention is featured in the provision of a multirate returnspring for a solenoid wherein the spring means comprises at least twosections having progressively greater winding pitches, such thatmulti-spring restoring force rates are employed which compositely can bemade to closely match the-solenoid force characteristic over apredetermined stroke.

These and other features and objects of the present invention willbecome apparent upon reading the following description with reference tothe accompanying drawing in which;

FIG. 1 is a diagrammatic representation of a solenoid member including areturn spring;

FIG. 1 is a mechancial schematic representation of a multirate spring inaccordance with the present invention; and

F l6. 3 is a diagrammatic representation of a solenoid forcestrokecharacteristic and a matching spring restoring forcedeflectioncharacteristic for an exampled embodiment in accordance with the presentinvention.

In many solenoid actuated devices, the return spring is required todrive an external load as well as return the plunger to its normalunenergized position. Certain devices employing solenoids require thatthe load be repeatedly positioned to precisely defined positions at therespective ends of the solenoid stroke. Such an application is exampledin my co-pending application entitled "Compact Annunciator Package"filed Oct. 14, 1969, Ser. No. 866,307, wherein is described anannunciator comprising a rotatable prism which carries information onthe surfaces thereof. Solenoids means are employed to rotate the prismto selectively present a desired one of the information bearing surfacesfor display. In such an application it is imperative that an energizedsolenoid position a load to a first precisely defined position and uponelectrical release, return the load to a second precisely definedposition. It is extremely desirable in such applications to maximize thespring return force for any given wattage solenoid employed. in general,the use of conventional single rate return springs in solenoids employedfor precise load positioning, fails to maximize the return force, and acompromise design employing a single rate return spring may fail to meetthe positioning specifications necessary for a given application.

As above described, .the force developed by a solenoid throughout theplunger stroke will in general vary inversely as the square of the airgap. The resulting inverse-square force relationship results in apronounced knee in the solenoid force characteristic which cannot bematched by a single rate spring. lt will become apparent fromthefollowing description that, while preloading techniques may improvethe return spring force characteristic such means alone do not optimizethe efficiency of the device. In accordance with the present inventionmeans are employed to match the spring return force characteristic withthe solenoid force characteristic such that throughout the strokeof asolenoid plunger substantially constant differential exists between thesolenoid force and the spring return force in the interest of improvingefficiency and maximizing spring return force at full stroke for a givenwattage solenoid.

With reference to FIG. 1, a spring return pull-type solenoid is depictedas comprising a body member 10 within which (not illustrated) a solenoidwinding is affixed which may be energized by application of power toterminals 8 and 9. With application of power to terminals 8 and 9, thesolenoid core member 7 is withdrawn into the body of the solenoid andagainst the restraining force of the compression spring member 16. Thesolenoid plunger 11 extends from the solenoid core 7 and is functionallydepicted as being attached to and driving a load member 12. Withdrawalof the plunger 7 within the body of the solenoid 10 develops a springrestoring force proportional to the deflection of the compression springmember, which deflection is exerted by lip 13 on the solenoid plunger 11forcing spring retainer 15 against the spring member 16 in a compressionmode. The illustrated solenoid in FIG. 1 is formed with a lip member 14,annular in form, which mechanically stops the spring retainer 15 suchthat a preloading of spring 16 may be realized by a predetermineddeflection of spring member 16 in the compression mode when the solenoidplunger 11 is in the illustrated unenergized or rest position. Thepreloading feature is normally utilized to establish a positive home"position of the plunger upon electrical release of the solenoid, and theamount of spring preloading is generally a design requirement for agiven system which depends on the characteristics of the positionableload member 12.

A typical solenoid force versus stroke characteristic is depicted inFIG. 3 by the curve 17 through points A, B, and C which exhibits aninverse-square characteristic which defines a pronounced knee in thevicinity of the points A and B. As above described, the inverse-squarecharacteristic is typical of solenoids since the force of a solenoidwill in general vary inversely with the square of the air gap. The homeor unenergized position of the solenoid is depicted at point A of curve17, corresponding to a stroke of 0.093 inches. The full energizedposition of the solenoid plunger is depicted as zero stroke at point Con curve 17, at which point a solenoid force of approximately 55 gramsis developed.

In accordance with the present invention a spring characteristic asdepicted by curve 18 of FIG. 3 is realized, wherein a substantiallyconstant force differential between the solenoid ositioning force andthe spring restoring force is maintained throughout the stroke. In theexampled embodiment discussed here, the spring characteristic is seen tobe comprised of two discretely different rate segments, the first ratebeing established between points A and B on curve 18 and a secondappreciably higher rate being established from the knee at point B tothe zero stroke point at C. Point A on curve 18 indicates a preloadforce of approximately 12 grams being exerted by the spring duringunenergized position of the solenoid plunger.

The dual rate spring restoring force characteristic of FIG. 3 isrealized by employment of a dual rate spring as functionally depicted inFIG. 2, wherein the spring 16 is comprised of a first section 16a havinga winding pitch'substantially less than that of a subsequent section16b. Point B on the spring characteristic of FIG. 3 corresponds to thatpoint during the solenoid stroke when the winding of lesser pitched coil16a of spring 16 close, with only the remaining greater pitched coil 16bbeing involved at a much higher rate. At the beginning of the stroke atpoint A, and including that portion of the characteristic from point Ato B, all the spring coils are opened and active and produce the initialrate as concerns the spring restoring force. The initial rate in theexampled embodiment is depicted as 0.505 lbl/in. between poirits A and BY on the spring characteristic 18. When the closer spaced coil 16a ofthe spring close at a stroke of 0.060 (point B) only the higher ratewider spaced coil 16b of the spring is involved and establishes thehigher spring rate of 0.853 lb./in. It is noted that the inverse-squaresolenoid force characteristic is 'very nearly approximated by theresulting spring characteristic which exhibits two distinctly differentrates throughout the solenoid stroke.

The solenoid and spring characteristics of FIG. 3 depict a moreefficient design than any design employing a single rate spring andadditionally allow a considerable flexibility as to solenoid design fora particular work application.

For a given load, a predetermined spring preloading may be necessary toassure a well-defined and positive return position when electricalrelease is initiated. The use of a too steep single rate spring withpreloading to maximize stored spring energy at the full stroke positionmay result in an intersect of the spring force-deflection characteristicwith the solenoid force- -stroke characteristic and prevent full strokeposition from being attained. Further, the use of too great a springrate on a single rate basis with preloading in a design with apredetermined preload may establish a desired return force from fullstroke at the expense of a solenoid force-spring restoring forcedifferential too small to facilitate desired plunger acceleration.

On the other hand, the employment of a single rate spring ofcomparatively steep characteristic, as in the steeper characteristicportion of the spring characteristic of FIG. 3, to avoid intersect withthe solenoid force displacement characteristic while maximizing springenergy at full stroke may establish too small a preload capability asdepicted by point A" in FIG. 3. Thus, while maximum spring energy may bestored at full stroke the use of this greater rate may impose a preloadlimit insufficient to positively and repeatedly define a stable returnplunger position for a given application.

The use of a single rate spring as defined by portion A-B' of curve 18of FIG. 3 would effect a full stroke restoring force of approximately 25grams in the exampled embodiment, far from the optimum maximum storedspring energy at electrical release that can be realized by themulti-rate spring embodiment of the present invention.

In the exampled embodiment therefor it is seen that the most efficientoperation for a given wattage solenoid is attained by closelyapproximating the inverse-square characteristic of the solenoid forcecurve 17, and this may be closely approached for any given solenoid bythe employment of a multirate spring. 1

Obviously in accordance with the present invention the spring member 16might be comprised of more than two sections in which case the solenoidforce characteristic might be more nearly and exactly approximated bythe spring characteristic. It is to be understood that when employingsprings having more than two sections, successive ones of the sectionswould have progressively greater winding pitches or be otherwisedesigned as to introduce predetermined spring rate segments in thespring loading characteristics the composite of which effect a match ofthe solenoid force characteristic as necessary for a given solenoid.

The present invention is thus seen to provide a flexible designcapability as concerns use of a given wattage solenoid in a workapplication. The solenoid efficiency for a given wattage rating isoptimized, and the spring return force maximized so as to enableprecisely defined solenoid plunger positions for a given loadapplication.

Although the present invention has been described with respect to aparticular embodiment thereof, it is not to be so limited as changesmight be made therein which fall within the scope of the invention asdefined in the appended claims.

lclaim:

1. In a solenoid of the type comprising a spring loaded plunger memberthe loading imposed'on which defines a predetermined spring return forceupon electrical release on said solenoid plunger member, means forstructuring said return spring to exhibit at least two progressivelygreater loading rates over the stroke of said solenoid plunger torealize a spring length-force characteristic over the stroke of saidsolenoid plunger the slope of which approximates that of the inherentinverse-square solenoid force-stroke characteristic, thereby maintaininga substantially constant differential between solenoid force and springretaining force over the stroke of said solenoid plunger whilemaximizing the retaining force of said return spring at the energizedterminus of the solenoid stroke.

2. A solenoid with multi-rate return spring as' defined in claim 1wherein said spring is comprised of multiple contiguous helical sectionshaving progressively greater winding pitches.

3. A solenoid with multi-rate spring return means as defined in claim 2wherein said multi-rate return spring comprises first and secondcontiguous sections having progressively greater winding pitches.

4. A solenoid with multi-rate return spring as defined in claim 2wherein the resilient characteristics of said spring member sections andthe respective comparative pitches thereof effect a closing of coils inthe lesser pitched sections of said spring at selected points within thestroke of said solenoid plunger so as to define successive greater ratespring forcesaid solenoid plunger stroke, said successive differentspring force-deflection rates forming a composite spring ratecharacteristic over the stroke of said solenoid substantiallyapproximating the slope of the inverse-square solenoid force-strokecharacteristic.

5. Means as defined in claim 4 wherein said spring return means iscompressibly confinedto exert a predetermined preload force in theunenergized position of said solenoid against which said solenoidplunger stroke is initially restrained upon energization of saidsolenoid.

6. Means for optimizing the etficiency of a spring return solenoidhaving a predetermined stroke versus force characteristic comprisingmeans for fashioning said return spring so as to exhibit successivelydifferent return rates over predetermined segments of said solenoidstroke to effect a spring force on said solenoid plunger with saidsolenoid unenergized to establish a predetennined diflerential over thestroke of said solenoid plunger between solenoid force imparted on saidplunger and the restraining force exerted on said plunger by said returnspring, said solenoid force being in excess of the restraining meansimparted by said spring member over the stroke of said solenoid.

* l I I III

1. In a solenoid of the type comprising a spring loaded plunger memberthe loading imposed on which defines a predetermined spring return forceupon electrical release on said solenoid plunger member, means forstructuring said return spring to exhibit at least two progressivelygreater loading rates over the stroke of said solenoid plunger torealize a spring length-force characteristic over the stroke of saidsolenoid plunger the slope of which approximates that of the inherentinverse-square solenoid force-stroke characteristic, thereby maintaininga substantially constant differential between solenoid force and springretaining force over the stroke of said solenoid plunger whilemaximizing the retaining force of said return spring at the energizedterminus of the solenoid stroke.
 2. A solenoid with multi-rate returnspring as defined in claim 1 wherein said spring is comprised ofmultiple contiguous helical sections having progressively greaterwinding pitches.
 3. A solenoid with multi-rate spring return means asdefined in claim 2 wherein said multi-rate return spriNg comprises firstand second contiguous sections having progressively greater windingpitches.
 4. A solenoid with multi-rate return spring as defined in claim2 wherein the resilient characteristics of said spring member sectionsand the respective comparative pitches thereof effect a closing of coilsin the lesser pitched sections of said spring at selected points withinthe stroke of said solenoid plunger so as to define successive greaterrate spring force-deflection characteristics from the unenergizedposition of said solenoid plunger stroke to the fully energized terminusof said solenoid plunger stroke, said successive different springforce-deflection rates forming a composite spring rate characteristicover the stroke of said solenoid substantially approximating the slopeof the inverse-square solenoid force-stroke characteristic.
 5. Means asdefined in claim 4 wherein said spring return means is compressiblyconfined to exert a predetermined preload force in the unenergizedposition of said solenoid against which said solenoid plunger stroke isinitially restrained upon energization of said solenoid.
 6. Means foroptimizing the efficiency of a spring return solenoid having apredetermined stroke versus force characteristic comprising means forfashioning said return spring so as to exhibit successively differentreturn rates over predetermined segments of said solenoid stroke toeffect a spring return force characteristic as a function of saidsolenoid stroke substantially approximating the slope of said solenoidforce-stroke characteristic.
 7. Means for optimizing as defined in claim6 wherein said return spring is preloaded to exert a predeterminedrestraining force on said solenoid plunger with said solenoidunenergized to establish a predetermined differential over the stroke ofsaid solenoid plunger between solenoid force imparted on said plungerand the restraining force exerted on said plunger by said return spring,said solenoid force being in excess of the restraining means imparted bysaid spring member over the stroke of said solenoid.